Field of the Invention
The present invention relates to a velocimeter that detects light modulated by a moving object with the Doppler effect and measures the velocity of the object, and also relates to a method of manufacturing an article.
Description of the Related Art
There has been used a Doppler velocimeter (hereinafter, also merely referred to as “velocimeter”) as an apparatus that measures the velocity of a moving object. A laser Doppler velocimeter (LDV) irradiates an object with laser light, and measures the velocity of the object by using the Doppler effect. The Doppler effect is an effect that the frequency (wavelength) of scattered light from an object shifts in proportion to the moving velocity of the object. It is known that a signal obtained by LDV typically has a low signal-to-noise (S/N) ratio. It is also known that factors affecting measurement accuracy may be mixing of high frequency noise and lowering of the level of a Doppler signal, or so-called dropout.
In Japanese Patent Laid-Open No. 8-15436, a Doppler signal, which is obtained by a light detector and from which noise is removed by a band pass filter, is compared with a reference level, and a level detection signal is output. Also, the Doppler signal is binarized and hence a periodic error (also referred to as a period error or an error of a period) is detected, and a period error signal is output. Then, an error signal is detected on the basis of the level detection signal and the period error signal.
However, the method of detecting the error signal in Japanese Patent Laid-Open No. 8-15436 determines an error only when both a level error and a period error are generated but does not determine an error when only one of these is generated to handle a signal with a rapid variation in frequency. Hence, the measurement accuracy may be decreased due to mixing of noise when a dropout is not generated.